USB all the way, baby.. It lets you move the adapter around to the spot where you get the best signal, they are cheaper, more portable, etc.
IMHO, PCI is not a good way to do wireless - You're sticking your wireless card INSIDE (and the antenna right out side of) a metal case filled with lots of things that interfere with wireless signals.
Get it well outside the case, move it around and you'll be much happier.
- G